Update extsprintf dependency

[danielbprice]

My build system is presently stuck behind a firewall, the misconfiguration of which is making it hard to get to github. We have a caching NPM proxy, which works well, and all of the modules we use during the build are presently cached. However, when node-jsprim pulls in a relatively old version of extsprintf (1.0.2), the git submodules initialization fails for git://github.com/... URIs. Those git submodules were removed from extsprintf in commit 76e9d14be8d6a jsPrintf dicey use of arguments. So updating the dependency would help us out in this (admittedly fringe) case.

I'm filing this issue and will followup with a PR to resolve it along with #21.

Happy to take guidance on other solutions, but this seemed to me to make sense.

[davepacheco]

Sure!

This repo uses a different contribution process now, but that was never updated in the README or a contributing file. I'll put together a change for this issue, #21, and to update the contribution guidelines.

[davepacheco]

Proposed change: https://cr.joyent.us/#/c/2317/1

This is make prepush clean.

[davepacheco]

Published in v1.4.1.